How to change one letter in file names throughout the file system?

I have hundreds of files with special characters ('æ', 'ø' and 'å') in the file names.

I cannot copy them to my external mntfs drive without renaming.

Files are in dozens of different folders. there are thousands of other files without these letters there.

I would like to replace special characters with my placeholders ('ae', 'oe' and 'aa'), while preserving the rest of the file name.

I am on ubuntu. I am thinking about using grep, sed and tr, but I don’t know exactly how to do this.